This invention relates to an injection timing control device for distributor-type fuel injection pumps, and more particularly to improvements in starting advance means provided in fuel injection pumps of this kind for advancing the injection timing at the start of the engine in a cold condition such as in cold weather.
An injection timing control device provided in distributor-type fuel injection pumps for diesel engines typically comprises a roller holder carrying a plurality of rollers circumferentially arranged and disposed in camming engagement with an end face of a pumping and distributing plunger connected to the output shaft of an engine, a timer piston coupled to the roller holder and arranged to be urged at its one end face by fuel pressure variable as a function of the engine speed supplied from the suction chamber of the pump, and a timer spring arranged to urge the timer piston against the fuel pressure. The timer piston is therefore displaceable in response to a change in the engine speed to cause a corresponding change in the circumferential position of the roller holder, which in turn causes a change in the axial operative position of the plunger relative to the circumferential phase of the engine output shaft to vary the injection timing.
In the conventional injection timing control device, due to rotative contact of a cam disc integral with the plunger with the rollers on the roller holder, a torque reaction force acts upon the rollers so as to cause circumferential vibration of the roller holder, which vibration is transmitted to the timer piston to cause axial vibration thereof. As a consequence, fuel is alternately introduced into and discharged from a hydraulic oil chamber defined at the one end of the timer piston, to amplify the axial vibration of the timer piston, resulting in unstable control of the injection timing.
To prevent such phenomenon, an injection timing control device for distributor-type fuel injection pumps has been proposed by U.S. Pat. No. 4,408,591 assigned to the same assignee of the present application, which is provided with a servo valve disposed to separate the hydraulic oil chamber defined at one end of the timer piston from the suction chamber to thereby prevent vibration of the timer piston attributable to a torque reaction force acting thereon, during operation of the engine.
On the other hand, injection timing control devices of the aforementioned type are generally provided with injection timing advance means for obtaining a required injection timing advance at the start of the engine so as to facilitate starting of the engine in a cold condition. Conventionally, such injection timing advance means include a type in which an auxiliary piston is urged by a spring having a setting load larger than a timer spring urging a timer piston, to bias the timer piston in an injection timing-advanced position at the start of the engine in a cold condition, as disclosed in Japanese Provisional Patent Publication No. 55-49078, and a type in which a roller holder of the injection timing control device is rotated in the injection timing advancing direction directly by a starting injection timing advance lever driven by an actuator, at the start of the engine in a cold condition, as disclosed in Japanese Provisional Patent Publication No. 58-163644.
However, these conventionally proposed injection timing advance means are large in size and complicated in construction, and therefore are not suitable for use in an injection timing control device equipped with a servo valve, referred to hereinbefore.